The term “fluid” includes liquid and gaseous products.
Two methods and systems for fluid transfer between vessels at the open sea are common: the so-called “Tandem System” and the “Side-by-Side System”.
When the Tandem System is used, the LNGC is typically located 80-150 m aft and “down weather” of the FLNG. As the vessels are kept at some distance from each other, the method is relatively safe from a collision point of view. Thus, the loading operation may be carried out under relatively high wave conditions.
As the LNGC normally has its manifold mid-ships, the cargo hoses, which may be aerial, floating or submerged, tend to be relatively long. A longer hose generally creates a higher pressure drop than a shorter hose, and it is often necessary to install increased pressure pumps in order to overcome the pressure drop. When loading LNG, more boil-off gas is generated with longer hoses and relatively large boil-off handling systems may be required.
When the Side-by-Side System is used, the LNGC is moored Side-by-Side relative the FLNG. The distance between them being only a few meters. The Side by Side System is relatively weather sensitive due to the risk of collision between the vessels.
The main advantage of the Side-by-Side System is that the distance between the manifold of the LNGC and the FLNG is relatively short. Traditional loading arms, short hoses, or similar that stretches through the air between the two vessels, may be used. Floating or submerged hoses exposed to waves and current are therefore not required. Since the length of the fluid transfer system is shorter, the pressure drop in the hoses are lower and standard pumps may be utilized. When loading LNG, the amount of boil-off gas is also significantly lower.
EP 2121462 shows a vessel having azimuth thrusters installed in order to improve manoeuvrability during loading operations.
In this document, the term “Parallel” configuration is utilized, mainly to distinguish the present method and system from the “Side-by-Side” configuration. The meaning of the term Parallel configuration is defined in the description below, and includes positioning a second vessel by the help of a self-propelled buoy relatively close to a first vessel.